1. Field of Invention
This invention relates to the detection of various gases in the atmosphere, and more specifically to measuring time weighted exposures of those gases with a chemically integrating dosimeter.
2. Prior Art Statement
High sensitivity determinations of various gases in the workroom atmosphere have been made necessary by the establishment of federal standards for industrial air. See, Federal Register, Vol. 36, No. 105, May 29, 1971. Although a wide variety of methods has been proposed and used for the determination of the many gases subject to these federal standards, the present invention relates specifically to the measurement of the following gases in workroom air at atmospheric pressures: hydrogen sulfide, H.sub.2 S; sulfur dioxide, SO.sub.2 ; hydrogen chloride, HCl; ammonia, NH.sub.3 ; hydrogen fluoride, HF; and hydrogen cyanide, HCN. The presence of these gases in sufficient quantities in industrial air can be injurious to human health. For example, hydrogen sulfide, H.sub.2 S, is a toxic contaminant often encountered in the industrial processing of gas and gas streams. Exposure to H.sub.2 S gas in even small amounts can result in olfactory paralysis in less than fifteen minutes. Longer periods of exposure result in sickness and death. See, Hydrocarbon Processing & Petroleum Refiner 42:115, April, 1963. Sulfur dioxide gas, SO.sub.2, is a major source of atmospheric pollution. Because of its corrosive and poisonous characteristics, sulfur dioxide is an extremely dangerous pollutant. The gas causes irritation and inflammation of the eyes and respiratory paralysis if present in sufficient concentration. Concentrations of approximately 1 ppm are believed injurious to plant life.
Threshold limit values (TLV), i.e., the time weighted average for maximum allowable exposure over an eight-hour day or a forty-hour week, have been prescribed for each of the gases listed above. See, "TLV's for Chemical Substances in Workroom Air," adopted by American Conference of Governmental Industrial Hygienists (ACGIH) for 1976. In particular, the current threshold limit value (TLV) for H.sub.2 S is 10 parts per million (ppm). Exposures up to 15 ppm are permissible for H.sub.2 S if the time weighted average remains below the TLV of 10 ppm. The current threshold limit value (TLV) for SO.sub.2 and HCl is 5 parts per million (ppm). Short term exposure is also limited to 5 ppm. The current TLV for ammonia, NH.sub.3, is 25 ppm. Short term exposure is limited to 35 ppm. The TLV for hydrogen cyanide, HCN, is 10 ppm, with allowable short term exposure at 15 ppm. The TLV for hydrogen fluoride, HF, is 3 ppm, with short term exposure also at 3 ppm.
Since these federal regulations require the measurement of time weighted exposure of workers to gases, techniques which make cumulative, i.e., time integrated, determinations of gas concentrations are desirable. Devices which measure concentrations at a particular instant are inconvenient since repeated measurements, followed by arithmetic integration, are required to arrive at a cumulative exposure value. Even devices which continually measure gas concentrations are limited by the need for arithmetic integration of results over time. Arithmetic integration of a series of measurements at specific time intervals is misleading if the interval between measurements is large enough that variations in gas concentrations are not detected. The expense of frequent measurements, coupled with the requirements for time integrated exposures, illustrates the limitations of present systems for determining long term exposure to gases.
One such present system, a means for measuring hydrogen sulfide gas, has been disclosed by Riseman, et al, and described in U.S. Pat. No. 3,915,831. A hydrogen sulfide, H.sub.2 S, sensing cell uses a sulfide-ion sensitive electrode, in conjunction with a reference electrode. The permeation of H.sub.2 S across a single membrane into a reference solution is determined by potentiometrically measuring the change in sulfide ion concentration in the solution.
One "Method of Determining Sulfur Dioxide and Sensing Cell Therefor" has been disclosed by Kreuger, Frant, and Riseman in U.S. Pat. No. 3,803,006. The method uses a hydrogen ion sensitive glass electrode in conjunction with a Ag/AgCl reference electrode. SO.sub.2 permeates across a single membrane into an aqueous solution of sulfite or bisulfite salt, where the SO.sub.2 dissolves and reacts with H.sup.+ ion to form sulfite and bisulfite ions. SO.sub.2 is determined by potentiometrically measuring the change in H.sup.+ ion concentration.
Chand has disclosed an apparatus for measuring sulfur dioxide in U.S. Pat. No. 3,622,488. SO.sub.2 permeates across a single semipermeable membrane into a dilute sulfuric acid electrolyte. Electro-oxidation of sulfur dioxide to SO.sub.4.sup.-2 occurs at a noble metal sensing electrode, and generated current is measured between this electrode and a counter-electrode. While SO.sub.2 concentration is thereby determined, the device does not measure time integrated exposure to SO.sub.2.
Reiszner and West describe a method for Determination of Sulfur Dioxide in "Environmental Science & Technology," Vol. 7, No. 6, p. 526, June 1973. SO.sub.2 gas permeates through a single gas permeable membrane into a sodium tetrachloromercurate (II) internal solution, forming the stable dichlorosulfitomercuate (II) complex. The dichlorosulfitomercurate (II) complex is extremely sensitive to direct sunlight and must be protected from solar radiation through the use of a light-shield mounting box. SO.sub.2 concentration is then determined by the lengthy and complex West-Gaeke procedure, described in ASTM 02914-70 T.
A measuring cell for determining the concentration of SO.sub.2 in a fluid has been disclosed by Dahms in U.S. Pat. No. 3,756,923. The cell includes an electrode covered with a thin layer of an electrolyte containing silver ions, and a counter electrode. When a voltage is applied across the electrodes, the resulting current is a measure of the concentration of SO.sub.2. In one of its forms, the electrolyte is separated from the sample fluid by means of a single membrane, composed of silicon rubber of polytetrafluorethylene. Disposed between the membrane and the electrolyte can be a porous spacer which is ion permeable and wettable. The function of the porous spacer is to provide a geometrically well-defined layer of electrolyte on the electrode. The cell does not make time integrated measurements of SO.sub.2 gas.
A sulfur oxide meter for measuring changes in SO.sub.2 activity directly in an electrochemical cell has been disclosed by Salzano, et al, in U.S. Pat. No. 3,718,546. Both a reference and a sample oxygen bearing electrode are exposed to a fused salt electrolyte. One or more membranes which are porous to a cation common to the electrolyte are used to isolate the reference and sample gas electrodes from each other. The SO.sub.2 activity is determined by measuring the output electromotive force (EMF) ofthe cell, which is a function of the difference in activities between the SO.sub.2 in the reference gas and that in the test sample. The device
For measurements of ammonia, NH.sub.3, an electrode has been disclosed by Riseman, et al, in U.S. Pat. No. 3,830,718. The standard electrolyte solution comprises a saturated aqueous solution of an ammonium salt of a strong acid having a pK of not more than 3, the salt having an aqueous solubility at room temperature such that the ammonium ion concentration is about 0.001 M to 1 M. A single microporous hydrophobic membrane with a porosity sufficiently great so as to readily pass ammonia gas but not great enough to permit any appreciable passage of liquid or ions, separates the electrolyte from the sample gas. The electrode provides real time, but not integrated, determinations of ammonia concentrations.
An ammonia sensor has been disclosed by A. Strickler, et al, in U.S. Pat. No. 3,649,505. An electrochemical cell comprises a hydrogen ion sensitive electrode and a reference electrode joined by an ammonium-ion containing electrolyte. The electrodes and electrolyte are separated from the sample being analyzed by a single microporous hydrophobic membrane, highly permeable to ammonia gas and substantially impermeable to liquid and ions. In a preferred form, a second inner hydrophilic membrane is interposed between the first membrane and the internal electrolyte. The second membrane is ion permeable and may be composed of very thin cellophane or filter paper. The inner membrane ensures that an electrolyte film is provided between the outer membrane and the ion sensitive electrode.
A dosimeter for measuring nitrogen dioxide has been disclosed by Ferber, et al, in U.S. Pat. No. 3,992,153, which makes arithmetically determined time weighted average measurements of NO.sub.2. The nitrogen oxide to be measured passes through a single, gas-permeable, liquid-impermeable membrane into an internal gas-absorbing solution. The rate of entry of gas molecules into the absorbing solution is controlled by the permeability of the membrane and by the concentration of the ambient gases. The gas molecules stoichiometrically react with the internal solution to form NO.sub.3.sup.- ion. The change in NO.sub.3.sup.- concentration is monitored with an ion sensor and the ambient NO.sub.2 gas concentration can be back calculated. Ferber also discloses the use of a glass-fiber filter impregnated with acidic sodium dichromate to convert NO to NO.sub.2 by oxidation. In this manner, exposure to NO can be determined. This filter is disposed externally to the membrane.
A gas-sensing electrochemical cell for measuring nitrogen dioxide dissolved in a sample solution has been disclosed by Kreuger, et al, in U.S. Pat. No. 3,830,709. The cell comprises a potentiometric hydrogen ion-sensitive electrode and a reference electrode, both in contact with an internal standard solution comprising an aqueous acid solution of a nitrite salt. A single hydrophobic gas permeable membrane separates the sample solution from the internal solution. The cell does not make time integrated measurements of NO.sub.2 gas.
Several other techniques for the general measurement of atmospheric gases have been disclosed in the art. For example, a polarographic sensor for measuring atmospheric gases, composed of a pair of electrodes joined by an electrolyte has been disclosed by Krull, et at, in U.S. Pat. No. 3,718,563. A multi-layer gas permeable, essentially ion impermeable membrane separates the electrodes and the electrolyte from the sample medium. The outer layer of the membrane is preferably formed of silicone rubber. The inner layer is formed of a material less permeable to gas and water vapor than the outer membrane.
Two other patents which relate to applicant's invention are described below. An electrochemical gas analyzer is disclosed by Laurer in U.S. Pat. No. 3,767,552. The anode and cathode are in contact with each other by means of an internal electrolyte. Separating the two electrodes is a disc, permeable to liquids but impermeable to solids, which prevents particles of the anode from contacting the cathode. A gas-permeable, liquid impermeable membrane separates the electrodes and the electrolyte from the sample to be analyzed. A second flexible, liquid impermeable expansion membrane is disposed internally to both the electrolyte and the electrodes.
An electrolytic sensor to measure carbon dioxide, CO.sub.2, with water diffusion compensation has been disclosed by Riseman, et al, in U.S. Pat. No. 3,357,907. A single membrane selectively permeable to gases separates an electrochemically active sample species from an electrode which is sensitive to an ionic concentration in an internal electrolyte placed between the electrode and the membrane. This ionic concentration is a fundtion of the concentration of the sample species. In one form of the invention, spacing means, such as a cellophane film, is disposed internally to the membrane. The spacing means is permeable to gases and water, and is wettable.